The comitatenses earn their existence to the military reforms of Constantine the Great. He divided the army into two types of troops: the other half of the system were the limitanei, or frontier troops. Originally, the comitatenses were regarded as an elite force personally asociated with the Emperor, but the needs of Imperial defence soon meant that they were detached as regional field armies across the Roman World. Enemies would be allowed to penetrate the "limes"(pronounced LEE-mays) or frontier line while being delayed by the limitanei, and then be destroyed in detail by the comitatenses.

Commanded by "comites"(the origin of the French aristocratic title "Comte", and therefore "Count"), these were well equipped and proffesional soldiers, even if not quite the equals of Caesar's legions.